The measurement of nanoscale deformations has various technological applications. Currently, the most important relates to the mapping of stresses in transistors based on strained silicon technology. This technology, which makes it possible to increase the working frequency of microelectronic devices, is presented in the document “Intel's 90 nm Logic Technology Using Strained Silicon Transistors”, M. Bohr, available on the Internet at the address: http://www.intel.com/research/downloads/Bohr-Strained-Silicon-120403.pdf.
The conventional methods of measuring deformations of a specimen, based on optical holography for example, do not allow nanoscale spatial resolutions to be attained. The techniques known from the prior art and having sufficient resolution are based on transmission electron microscopy, either in diffraction mode or in imaging mode. These techniques are described in a general manner in the article by B. Foran et al., “Strain Measurement by Transmission Electron Microscopy”, Future Fab International, 20 (2006) 127.
Among the techniques based on electron diffraction it is possible to mention CBED (Convergent Beam Electron Diffraction), LACBED (Large Angle Convergent Beam Electron Diffraction) and NBD (Nano-Beam Diffraction). The first two techniques have the drawback of providing information about deformation of the specimen studied only through comparison of the measurement data and the results of a simulation: the measurements are therefore indirect and depend heavily on the parameters chosen for the simulation. The latter technique lacks precision. Furthermore, these techniques meet with difficulties when the gradient of the deformations is too great, for example in the channel of a strained silicon transistor. In addition, the measurements are punctiform, or at most carried out in a row, and must therefore be repeated several times to allow the deformations of a specimen to be mapped in two dimensions.
The imaging technique based on diffraction contrast (QEDC) suffers from similar drawbacks.
By contrast, other imaging techniques, such as High Resolution Transmission Electron Microscopy (HRTEM), have the advantage of being “direct” and of not depending on a particular choice of simulation parameters. However, these techniques only provide an image of the crystal lattice at the nanoscale; determining a state of deformation assumes the availability of a reference lattice. Yet in order to image a crystal lattice, it is necessary to enlarge it very greatly (by a factor of around 5×105), which means that the field of view is necessarily narrow (of around 100-150 nm). It is therefore not possible, in general, to view at the same time a deformed region and a region without deformation that is able to serve as a reference.
Furthermore, all these techniques (and more particularly HRTEM) work satisfactorily only with very thin specimens, a few tens of nanometers thick at most for HRTEM. Yet, when a wafer that thin of a bulky structure is extracted, the stresses relax significantly. The deformations measured in the specimen are therefore no longer representative of those present in the original structure. This problem is well known in the art: see, for example, the article by M. M. J. Treacy et al. “On Elastic Relaxation and Long Wavelength Microstructures in Spinoidally Decomposed InxGa1-xAsyP1-y Epitaxial Layers”, Philos. Mag. A 51 (1985) 389.